Masking material particularly for gas plating processes



Jan. 31, 1961 w. M. BOLTON 2,970,064

MASKING MATERIAL PARTICULARLY FOR GAS PLATING PROCESSES Filed May 15,1957 FIG. I

7 7 r1 II II II w ll I! II IN VEN TOR. W/LBUR M. BOLTON ATTORNEYS UnitedStates Patent 9 MASKING MATERIAL PARTICULARLY FOR'GAS PLATING PROCESSESM. Bolton, Piqua, Ohio, assignor, by mesne assignrneiits; to UnionCarbide COT'POI'QUOHyNGW York, N.Y., a corporation of New York Filed May13, 1957, Ser. No. 658,568

4 Claims. (Cll 117-55) 1 This invention relates to the metallizing ofmaterials by"n'iear'is"-of'depos'ition of metal from the gaseous stateis particularly concerned with the masking off of selected areas of asubstrate to inhibit metallic deposition' thereon; further, theinvention is specifically conclei'ned w'ith'masking agents which notonly inhibit the nietallic deposition but which are readily removed froma substrate after completion'of metallizing and which provide a's'harp'line of demarcation between a metalliaed and a non-metallized area.

he invention has been found to be of considerable utility inItheproductionof printedcircuits wherein an electrically" conductive film ofmetal of relatively narrow u'fidth'and frequently serpentineconfiguration is provided oii'a substrate of insulating materiaL'such asa board of zf'phenolic' resin, for exam le.

li ac-c rdance with thisinvention a masking agent wlilehexhibitsparticular utility in gas plating applicatitiiisf, including the platingof printed circuits, comprises afilmfformer and a vehicle of lowvolatility. With such cfonib ination it appears that in the course ofgas plating the ve'hicleis slowly released from the filmand'crea'tesover the film avapor pressure of the vehicle which inhibits deposition-of the metal from the gaseous'constituent on the film. With suchdeposition inhibitor present d efpos'itior' may be substantiallycompletely prevented or'ifit does'oc'cur, the deposition is of suchadiscontinuotis nature or is soslight that penetration of the film bit'a subsequently applied washing liquid is" not preven a; I

Theffilm itself is preferably of such a nature that it may be removed,with anyresidual vehicle therein, from th substrate" without damage tothe substrate by' corn in I washing-liquids, such as water, soapsolutions; water and" alcohol solutions, and similar materials.

I";hayefound that film formers which are not reactive with the vehicleto produce material'sinsoluble in comtrion w'ashing liquids'areparticularly effective; such film formers also are suitablywater-soluble or water disp'ei-fsibl'e and are'not rendered insoluble bymoderate tem pei'atures up to 400-500 F. Particularly etfective for thepr'a'ctice of the invention are film formers such as plant; vegetableand bacteriological gums; for example,

aesaraat; celluloses such as carboxy methyl cellulose,

and alginates are useful. i

The vehicle for combination with the film former is suitably apolyhydroxy aliphatic, such as glycerin, ethylene glycol, propyleneglycol and high analogues, and also including'theglycol ethers and mixedethers, for example; butycarbitol is a useful ether.

For ease of practice of the invention I have found that'avehicle whichisreadily miscible with water, that is gly'cerine, is to'b e preferred;Glycerine is readily retained by film formers such as dextran at normaltemtui'e'sat sucha rate as to admirably perform the necesdeposition-inhibitingfunction; In combination withice 2. dextran andwater glycerine forms a convenient dispersion for film forming.

The invention will be more fully understood by reference to thefollowing detailed description and accompanying drawings in which:

Figure l is a schematic view illustrating apparatus useful in thepractice of the invention;

Figure 2 is a plan'view of a board of insulating material masked ofi forthe formation of a printed circuit utilizingthe apparatus of Figure 1;

Figure 3 is an enlarged and sectional view of the board' of Figure 2taken substantially on line 3-3 of Figure 2;

Figure 4 represents thestructure of Figures 2 and 3 in the course of themetallizing operation indicating sche matically the vaporization ofvehicle from the'masking film;

Figure 5 is a view similar to that of Figure 3 after metallizingand-before removal of the masking agent; and

Figure 6 is a plan view of the product of the operation in full scaleand after removal of the masking agent.

Referring to the drawings and initially particularly to Figure 2 thenumeral 1 designates a thin rectangular board of electric'al'insulatingmaterial such as a phenolic resin base board. Coated on the board 1 andoutlininga configuration 3lfor a printed circuit which is to includeterminals as at 5 is a masking agent 7 produced in accordance with theprinciples of this invention.

An excellent masking agent comprises as the filmformer, dextran a highmolecular weight gum glycerinas the vehicle of 10w volatility, and wateras the dispersing agent; While the water content mayvarywith themolecular weight of the gum andmay be varied to aid ready disp ersibility a convenient formula well suited" to the purpose has been foundto be approximately equal weights of glycerin and dextran together withwater to the'extent of'about'IO'times the dextran weight; Thus a formulaemploying 10 grams of dextran (M.W. approx. 1,000,000), 10 gramsofglycerin' and cc. of water has been employed The board 1 prior tocoating should 'be cleaned of oil or grease chemically and suitably theboard is also roughened by sand-blasting or with a 320 grit emery. v Thedispersion is spread on the cleaned board 1 in any convenient manner toform the film and to outline the" and'then a secondcoat is applied anddried at the same temperature. Such is a most suitable procedure forthe;

specific formulation noted above. The drying in each instance shouldtakeplace until the exposed surface of the film is just tacky.

The board with the film formed thereon is then placedin the apparatus ofFigure 1, for example, where it is exposed to an atmosphere of a platinggas; such may suitably' be nickel carbonyl and the temperature of theboard within the chamber is then brought up to 285 to 290 F. as will benoted hereinafter in connection with the specific description of theapparatus.

As the board 1 and film 7 are heated glycerin slowly;

evolves as indicated at 9 in Figure 4 and-a vapor pressure of glycerinexists over the masked areas. This vaporpressure tends to preventdeposition of the metal of theni'ckel carbonyl on the masked areas butdoes not prepent the build-up of nickel in the unmasked areas as-at 11(Figures 3, 4 and 5').

I With the masking agent specifically referred .to'above an exposuretime of ten minutes to the plating'gas has Patented Jan. 31, 1961 whichis desirable in aiding the final-washing. The wash-- ing may be effectedwith plain water and such is generally preferable; however, as alreadynoted, was liquids which do not affect the insulating board 1 or thedeposit of metal may be employed.

The masking agent is completely removable and it leaves a finelydelineated circuit configuration (Figure 6). Referring now moreparticularly to the exemplary apparatus illustrated in Figure 1, thenumeral 13 designates a vaporizing chamber, while the numeral 15indicates a conduit extending into the chamber, terminating in a nozzle17. and which is adapted for the flow of nickel carbonyl to the chamberfrom a suitable source of liquid carbonyl indicated at 19.

Leftwardly in Figure 1 the numeral 21 designates a conduit which isconnected to a source of helium, while the numeral 23 designates aconduit which is connected to a source of anhydrous ammonium gas. Valves25, 27 are provided in the respective conduits for the control of gasfiow.

The helium and ammonia mix together in the conduit 29 and flow to thechamber 13, where they mix and serve as a carrier for the nickelcarbonyl gas formed by the flash vaporizing of the liquid nickelcarbonyl. This plating gas then passes through conduit 31 to the platingchamber 33 which, in its lower portion, is provided with a platen 35adapted to be heated electrically through leads 37, 39. As indicated inFigure 1 the insulating board 1 is placed on and heated from the platen35.

Gases are exhausted from the plating chamber 33 through conduits as at41, 43, 45. These exhaust gases may suitably be flowed to a burner or toa recovery apparatus as desired.

In connection with the plating operation it is desirable that theplating chamber be of sufficient size, such that no build-up of pressureof the vehicle will occur over the areas which are to be metallized.This is not a critical matter and obviously the volume should be suchthat no material amount of glycerin, for example, exists in the vaporpressure within the plating chamber, the primary vapor pressure of thevehicle existing only at the masked area as it emanates from the maskingfilm.

When dextran is used as the film former, glycerin may be present to theextent of about percent of the weight of the dextran, or glycerin may bepresent in considerable excess, for example, ten times that of theweight of the dextran.

Similar considerations apply to the use of the other film forming agentswith respect to proportions. However, as already noted, it is preferablethat the glycerin be present in the masking agent to such an extent thatunder the conditions in which it is to be employed, some glycerin willremain in the film after the completion of the metallizing operation.

In some instances some slight deposition of metal has occurred on themasking agent, but in every case this deposition has been in such astate that it is readily permeable by water, and no difliculty hasbeenlexperienced in removing the films with water alone, although thespeed of this operation may be increased by employing a slight alkali,such as a soap solution. Also, if desired, an alcohol-water mixture tothe extent of about 50 percent alcohol, may be employed.

The success of this method of plating is attributed to the non-coherencyof any metal deposit which occurs on the masking film, thus providingfor the penetration of water, while the metal deposit itself which formsthe conduction portion of the printed circuit is highly coherent.

While large amounts of glycerin are employable, it is preferable tooperate on the side of the minimum of vapor pressure, and hence a smallamount of glycerin, in order to avoid the formation of slight haloswhich tend to prevent sharp delineation of the metal deposits.

Accordingly, while the vehicle should be present in sufficient extent tohavesome glycerin remaining in the film at the end of the operation, itshould not be present to such an extent that undue halo formationoccurs; When the amount of glycerin exceeds the proportions set out inthe ranges given hereinbefore, such halo formation may occur and affectsthe appearance of the article but does not deleteriously affect theutility.

It will be understood that this invention is susceptible to modificationin order to adapt it to difierent usages and conditions and accordingly,it is desired to comprehend such modifications within this invention asmay fall within the scope of the appended claims.

I claim:

1. A process for the production of well delineated metallic depositscovering only a surface portion of an object which comprises applying tothe object a masking agent to delineate on the object the contour of thedeposit to be formed, said masking agent comprising a film formerconsisting of a viscous mass composed of dextran and a glycol vehiclereleasable from the film former at temperatures below 500' F., said filmformer being water dispersible and capable of retaining its waterdispersibility when subjected to heat, heating the object in an enclosedvessel to a temperature sufficiently high to decompose a metal-bearingheat decomposable gaseous compound and less than 500 F. to inducevolatilization of the vehicle and to create a vapor pressure of thevehicle over the masking agent, passing a heat-decomposable gaseouscompound which is decomposable at temperatures above 75 F. and below 500F. to the object to be plated while the object is maintained atdecomposition temperature and while said vehicle is being volatilized tothereby deposit on the delineated portion of the object metal while thedeposition of metal on the masking agent is inhibited, removing theobject from the vessel, and thereafter washing the masking agent fromthe object.

2. A process for the production of well delineated metallic depositscovering only a surface portion of an object which comprises applying tothe object a masking agent to delineate on the object the contour of thedeposit to be formed, said masking agent comprising a film formerconsisting of a viscous mass composed of dextran and glycerine, heatingthe object in an enclosed vessel to a temperature sufficiently high todecompose a metalbearing heat-decomposable gaseous compound but to lessthan 500 F. to induce volatilization of the glycerine and to create avapor pressure of the vehicle over the masking agent, passing a saidheat-decomposable gaseous compound to the object to be plated while theobject is maintained at said decomposition temperature and while saidvehicle is being volatilized to thereby deposit on the delineatedportion of the object metal while the deposition of metal on the maskingagent is inhibited, continuing the deposition to produce a desiredplating thickness, removing the object from the vessel while someglycerine remains in the masking agent, and thereafter washing themasking agent from the object.

3. A process for the production of well delineated metallic depositscovering only a surface portion of an object which comprises applying tothe object a maskng agent to delineate on the object the contour of thedeposit to be formed, said masking agent comprising a film formerconsisting of a viscous mass composed of dextran, glycerine and water,preparing the masking agent by heating a film thereof on the object todrive off water until the film is just tacky, heating the object in anenclosed.

vessel to a temperature sufficiently high to decompose a metal-bearingheat-decomposable gaseous compound toto be plated while the object ismaintained at said decomposition temperature and while said vehicle isbeing volatilized to thereby deposit on the delineated portion of theobject metal while the deposition of metal on the masking agent isinhibited, continuing the deposition to produce a desired platingthickness, removing the object from the vessel while some glycerineremains in the masking agent, and thereafter washing the masking agentfrom the object.

4. A process for the production of well delineated metallic depositscovering only a surface portion of an object which comprises applying tothe object a masking agent to delineate on the object the contour of thedeposit to be formed, said masking agent comprising a film formerconsisting of a viscous mass made up of d'extran and glycerine the samebeing in approximately equal amounts by weight dextran and glycerine,heating the object in an enclosed vessel to a temperature suflicientlyhigh to decompose a metal-bearing heat-decompossble gaseous compound toinduce volatilization of the glycerine and to create a vapor pressure ofthe vehicle over the masking agent, passing a said heat-decomposablegaseous compound to the object to be plated while the object ismaintained at said decomposition temperature and while said vehicle isbeing volatilized to thereby deposit on the 6 delineated portion of theobject metal while the deposition of metal on the masking agent isinhibited, While glycerine is retained in the masking agent removing theobject from the vessel, and thereafter washing the masking agent fromthe object.

References Cited in the file of this patent UNITED STATES PATENTS1,656,265 Cosner Jan. 17, 1928 2,139,640 Mall Dec. 6, 1938 2,306,887Klose Dec. 29, 1942 2,484,019 Devores Oct. 11, 1949 2,602,755 SilvermailJuly 8, 1952 2,653,108 Oakley Sept. 22, 1953 2,668,120 Leaf et a1. Feb.2, 1954 2,674,584 Deniston Apr. 6, 1954 2,716,615 Voris Aug. 30, 19552,736,710 Nack et a1. Feb. 28, 1956 2,835,603 Swinehart et al May 20,1958 2,847,330 Toulmin Aug. 12, 1958 OTHER REFERENCES Printed CircuitTechnique, National Bureau of Standards Circular 468, issued Nov. 15,1947, pages 25 and 26.

1. A PROCESS FOR THE PRODUCTION OF WELL DELINEATED METALLIC DEPOSITSCOVERING ONLY A SURFACE PORTION OF AN OBJECT WHICH COMPRISES APPLYING TOTHE OBJECT A MASKING AGENT TO DELINEATE ON THE OBJECT THE CONTOUR OF THEDEPOSIT TO BE FORMED, SAID MASKING AGENT COMPRISING A FILM FORMERCONSISTING OF A VISCOUS MASS COMPOSED OF DEXTRAN AND A GLYCOL VEHICLERELEASABLE FROM THE FILM FORMER AT TEMPERATURES BELOW 500*F., SAID FILMFORMER BEING WATER DISPERSIBLE AND CAPABLE OF RETAINING ITS WATERDISPERSIBILITY WHEN SUBJECT TO HEAT, HEATING THE OBJECT IN AN ENCLOSEDVESSEL TO A TEMPERATURE SUFFICIENTLY HIGH TO DECOMPOSE A METAL-BEARINGHEAT DECOMPOSABLE GASEOUS COMPOUND AND LESS THEN 500* F. TO INDUCEVOLATILIZATION OF THE VEHICLE AND TO CREATE A VAPOR PRESSURE OF THEVEHICLE OVER THE MASKING AGENT, PASSING A HEAT-DECOMPOSABLE GASEOUSCOMPOUND WHICH IS DECOMPOSABLE AT TEMPERATURES ABOVE 75*F. AND BELOW500*F. TO THE OBJECT TO BE PLATED WHILE THE OBJECT IS MAINTAINED ATDECOMPOSITION TEMPERATURE AND WHILE SAID VEHICLE IS BEING VOLATILIZED TOTHEREBY DEPOSIT ON THE DELINEATED PORTION OF THE OBJECT METAL WHILE THEDEPOSITION OF METAL ON THE MASKING AGENT IS INHIBITIED, REMOVING THEOBJECT FROM THE VESSEL, AND THEREAFTER WASHING THE MASKING AGENT FROMTHE OBJECT.